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The term meta denotes a concept of transcendence or overcoming, and when applied to materials, metamaterials include artificially engineered substances that exhibit properties not found naturally in the environment. Metasurfaces, characterized by their thinness and lightness, have garnered considerable interest as a potential component for incorporation into portable augmented reality (AR) and virtual reality (VR) devices to facilitate holographic generation. However, it is important to note that metasurfaces have inherent limitations, such as their limited ability to store information and their ability to generate holograms exclusively within the visible spectrum.
The research team composed by Professor Junsuk Rho of the Department of Mechanical Engineering and Department of Chemical Engineering and Joohoon Kim of the Department of Mechanical Engineering of Pohang University of Science and Technology (POSTECH) has achieved the generation of meta-holograms applicable to both the regions spectral ranges of the visible and ultraviolet. The research results were published in Nanoscale horizons.
The restriction of hologram generation to the visible spectral range has been primarily attributed to the light absorption exhibited by most objects in the ultraviolet regime. However, the research team effectively addressed this challenge by incorporating a thin layer of specially formulated gas compositions into the metasurfaces, thus achieving significant improvements in holographic transmission efficiency in both the visible and ultraviolet regimes.
Furthermore, the team completed encoding two distinct holographic phase profiles on a single metasurface. The polarization characteristics of light regulate its propagation in space. Exploiting this phenomenon, the team’s approach enables the provision of holographic information for both clockwise and counterclockwise circularly polarized light, effectively doubling the amount of information encoded on metasurfaces.
To facilitate practical implementation, the team employed liquid crystals, a component commonly used in mobile phones and LCD displays, which allows for convenient manipulation of the rotational direction of light. Experimental results have shown that in the absence of an electric field, the light exhibits a clockwise rotation, thus generating an A-type hologram. Conversely, the application of an electric field induces a different direction of rotation of the light, with resulting in the generation of a type B hologram. Essentially, the research team designed a device that could present distinct holograms based on the presence or absence of an electric field.
Professor Junsuk Rho, who led the research, underlined this breakthrough by commenting, ‘This study is significant as it overcomes the limitations associated with meta-holograms applicable solely to the visible regime and we have achieved the simultaneous generation of meta-holograms in both the visible than in the UV domains.” He added: ‘This proposed metasurface may have promising applications in security technologies such as anti-counterfeiting measures, identifications and passports.’
Aqsa Asad et al, spin-isolated ultraviolet-visible dynamic meta-holographic displays with liquid crystal modulators, Nanoscale horizons (2023). DOI: 10.1039/D2NH00555G
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